1. Field of the Invention
The present invention relates to a power control device and a power control method for controlling the transmission power of a signal to be transmitted between a mobile station and a base station in a mobile communication system via a radio channel, and more particularly to a power control device and a power control method for controlling the transmission power by adjusting a target power value to be the target of the transmission power of signals which are transmitted via a radio channel which is set between a mobile station and a base station which communicates with the mobile station.
2. Description of the Related Art
As a Code Division Multiple Access (CDMA) type mobile communication system, a cdmaOne type mobile communication system has been commercialized, and a cdma2000 type mobile communication system which uses a broader bandwidth will be commercialized in the near future.
In a CDMA system, since a plurality of mobile stations (e.g. cellular phones, car phones) use a same frequency band, the interference power between mobile stations must be decreased in order to increase the number of multiple mobile stations. To achieve this, power control for controlling the transmission power of a mobile station to an optimum value is performed.
There are two types of power control: closed loop power control and outer loop power control.
Closed loop power control controls the transmission power of a mobile station by a base station, which measures the transmission power of a pilot channel or fundamental channel transmitted from a mobile station, and compares the transmission power and a predetermined target power threshold value (target power value) so that the transmission power becomes or approaches the target power threshold value. From the base station to the mobile station, power control bits are transmitted to instruct an increase/decrease of the transmission power, and the mobile station increases/decreases the transmission power according to the transmitted power control bits.
Outer loop power control, on the other hand, changes (adjusts the increase/decrease of) the target power threshold value by the base station or by the base station control device, which calculates the error rate (FER: Frame Error Rate) of frames transmitted from a mobile station via a fundamental channel, and compares this calculated FER with a predetermined target FER value (fixed value) so that FER becomes or approaches the target FER value. When the target power threshold value is changed, closed loop power control is performed based on the changed target power threshold value.
The fundamental channel generally carries call voice data, which is continuous data. Therefore, on the fundamental channel, a communication unit with a predetermined length, referred to as a frame, is always generated, and data is transmitted in frame units. Even in a silent state, frames to carry minimum-rate data (silent data) are generated and transmitted on the fundamental channel. In the cdmaOne system, for example, frames having time length of 20 [msec] are continuously generated on the fundamental channel.
Therefore, in the case of conventional outer loop power control, N=100 (=2 [sec]÷20 [msec]) frame units are received in a predetermined measurement period T=2 [sec] units, for example, and the number M of frames, which have frame errors included in the received frames, is counted. Then FER is calculated by determining FER=M÷N. And by comparing the calculated FER with a target FER value, which is set to 1%, for example, the change of the target power threshold value is controlled.
In this way, conventional outer loop power control is performed based on the fact that frames are always generated on a channel, and the number of frames to be received within a predetermined measurement period T is always constant.
However, in the case of a channel to carry data which is not transmitted continuously, such as packet data, frames need not be generated all the time on a channel, and it is efficient to generate frames only when data is transmitted. In the cdma2000 system, for example, a supplemental channel is disposed as a channel to transmit such discrete data as packet data, and in the supplemental channel frames are generated only when data is transmitted.
In such a case, if conventional outer loop power control is used, the accuracy of control may deteriorate, or control may substantially malfunction.
In other words, in order to implement a 1% accuracy when the target FER value is 1%, for example, at least 100 frames must be received to calculate FER. In the case of a supplemental channel, however, 100 frames are not always received within the measurement period T=2 [sec]. If FER is calculated with less than 100 frames and the result is compared with the target FER in this case, accuracy of control deteriorates.
Also when it takes several minutes or several tens of minutes to receive 100 frames, for example, outer loop power control does not function during this time. Also, 100 frames may not be received by the end of communication of a mobile station, and in this case outer loop power control is not performed.